1. Field of the Invention
The present invention relates to an apparatus for driving a brushless DC motor, and more particularly to an apparatus for driving a brushless DC motor which does not require a position sensor for detecting the rotating position of a permanent magnet rotor.
2. Description of the Prior Art
The brushless DC motor is longer in life and lower in electric noise level as compared with a DC motor with brushes because it has no mechanical contact. It is accordingly used widely in industrial equipment and video and audio equipment where high reliability is required.
Conventionally, the brushless DC motor of this type used a position sensor (for example, a Hall effect element), which corresponds to the brush, for changing over the conductive phases of the stator windings. The position sensor itself is not inexpensive, and it requires a complicated adjustment of the mounting position of the sensor and an increase in the total amount of wiring, which results in a much higher cost of the brushless DC motor as compared with a DC motor with brushes. Besides, the motor structure is often limited because the position sensor must be assembled in the motor. Recently, in the downsizing trend of appliances, the motor is becoming smaller and thinner, and a position for mounting the position sensor such as the Hall element, is not sufficiently available. Accordingly, several brushless DC motors without position sensors have been so far proposed.
As a brushless DC motor without a position sensor, for example, a motor using an output signal of a frequency generator attached to the motor is known. That is, the output pulses of the frequency generator for generating pulses depending on the rotation of the rotor are counted by a counter, and drive currents of a preset current pattern are sequentially passed to three-phase stator windings depending on the count value, thereby rotating the permanent magnet rotor (for example, see Japanese Patent Publication No. 63-262088).
In such a configuration, however, since the initial position of the rotor is not known when turning on the power, a special reset signal generating circuit is provided in the brushless DC motor as shown in the above prior art, and the counter is reset by the reset signal when the power is turned on, while a specific reset current is supplied to the stator windings so that the rotor and the stator windings may be preliminarily set in a specific positional relationship.
However, when the specific reset current is supplied to the stator windings in order to determine the initial position, the rotor begins to rotate, and the position of the rotor oscillates about the specified position, and not standing still at the specified position for a short time. As a result, the starting time is long because the operation cannot be transferred in a short time from the reset mode for supplying the specific reset current to the stator windings when turning on the power to stop the rotor at the specified position, to the normal position detecting mode for counting the output pulses of the frequency generator depending on the rotation of the rotor.
It cannot be therefore used in applications requiring a short starting time such as machines repeating rotating and stopping frequently.
In the brushless DC motor in the above prior art, since the rotor is rotated forcedly to a specified position in the reset mode, the rotor may largely rotate reversely depending on the initial position of the rotor. Hence, it can be used only in machines allowing a wide reverse rotation, and the scope of application is considerably limited.
Furthermore, in the brushless DC motor in the above prior art, if the relative positions of the rotor and stator windings are set in the specified relationship in the reset mode, if the rotor is loaded, the relative positions of the rotor and stator windings may vary largely depending on the magnitude of the load. Hence, the rotor may not be fixed at the specified position in the reset mode.
Therefore, in the brushless DC motor shown in the above prior art, when the reset mode is transferred to the normal position detecting mode for counting the output pulses of the frequency generator depending on the rotation of the rotor, the current phase supplied to the stator windings is largely deviated from the normal phase, and a high efficiency drive cannot be realized. Therefore, the brushless DC motor shown in the prior art is usable only in an application in which the motor is unloaded when the power is turned on.